1. Field of the Invention
The present invention relates to a hard film to improve the wear resistance of cutting tools such as tips, drills, and end mills, a cutting tool coated with said hard film which exhibits excellent wear resistance, a process for forming said hard film, and a target used as a vapor source to form said hard film.
2. Description of the Related Arts
It has been common practice to coat cutting tools made of cemented carbide, cermet, or high speed tool steel with hard film of TiN, TiCN, TiAlN, or the like for the purpose of improving their wear resistance.
Because of its excellent wear resistance as disclosed in Japanese Patent No. 2644710, the film of compound nitride of Ti and Al (referred to as TiAlN hereinafter) has superseded the film of titanium nitride, titanium carbide, or titanium carbonitride to be applied to cutting tools for high speed cutting or for high hardness materials such as quenched steel.
There is an increasing demand for hard film with improved wear resistance as the work material becomes harder and the cutting speed increases.
It is known that the above-mentioned TiAlN film increases in hardness and improves in wear resistance upon incorporation with Al. Japanese Patent No. 2644710 indicates that TiAlN precipitates soft AlN of ZnS structure when the Al content therein is such that the compositional ratio x of Al exceeds 0.7 in the formula (AlxTi1−x)N representing TiAlN. The foregoing patent also mentions that “if the Al content (x) exceeds 0.75, the hard film has a composition similar to that of AlN and hence becomes soft, permitting the flank to wear easily”. In addition, the foregoing patent shows in FIG. 3 the relation between the compositional ratio of Al and the hardness of film. It is noted that the hardness begins to decrease as the compositional ratio of Al exceeds about 0.6. This suggests that AlN of ZnS structure begins to separate out when the compositional ratio of Al is in the range of 0.6–0.7 and AlN of ZnS structure separates out more as the compositional ratio of Al increases further, with the result that the hardness of film decreases accordingly. Moreover, the foregoing patent mentions that the TiAlN film begins to oxidize at 800° C. or above when the compositional ratio x of Al is 0.56 or higher, and this temperature rises according as the value x increases. The temperature which the TiAlN film withstands without oxidation is about 850° C. when the compositional ratio of Al is 0.75 (which is the upper limit for the TiAlN film to have adequate hardness).
In other words, the conventional TiAlN film cannot have both high hardness and good oxidation resistance because there is a limit to increasing hardness by increasing the compositional ratio of Al. Consequently, it is limited also in improvement in wear resistance.
At present, cutting tools are required to be used at higher speeds for higher efficiency. Cutting tools meeting such requirements need hard coating film which has better wear resistance than before.